A read only memory (ROM) consists of an array of semiconductor devices (diodes, bipolar or field-effect transistors) which interconnect to store an array of binary data (ones or zeros). A ROM basically consists of a memory array of programmed data and a decoder to select the data located at a desired address in the memory array.
Though there are three basic types of ROMs, mask-programmable ROMs, erasable programmable ROMs (EPROMs) and field-programmable ROMs (PROMs), the focus of the present invention is on PROMs.
PROMs are typically manufactured with all switching elements present in the array, with the connection at each row-column intersection being made by means of either a fuse element or an antifuse element. In order to store data in the PROM, these elements (either the fuse or the anti-fuse, whichever are used in the design) are selectively programmed using appropriate voltage pulses supplied by a PROM programmer. Once the elements are programmed, the data is permanently stored in the memory array.
However, the programming pulse required to rupture an antifuse element is normally in the neighborhood of 14-20 V. If this programming pulse can be reduced the semiconductor device would therefore be subjected to a less potentially damaging voltage. Ideally, the closer the programming pulse gets to normal operating voltages of a given device the better as larger devices normally required to handle large voltages can be reduced in size thereby reducing the potential problems of transistor punch through, gate oxide breakdown, etc.
The present invention focuses on substantially lowering the programming pulse by developing an antifuse element overlying a local field enhanced diffusion region.